Solar cells find extensive application in space-related vehicles for providing power from the sun. Obviously, high efficiency solar cells, such as multijunction (MJ) cells, are preferably utilized; however, such high efficiency solar cells tend to be expensive. Due to the high overall expense of satellite launch, deployment, and operation in space, it is desirable to extend the lifetime of the solar cells, whether in earth orbit, at various altitudes, or on an interplanetary mission. It is further desirable to provide a concentrator for solar cells in space environments that yields higher concentration ratios (on the order of 50.times. or more), while at the same time providing shielding against charged particles (mostly electrons and protons) within the energy range of a few KeV to 100 MeV. Further, such a concentrator must be light-weight and compact and must not require excessive solar tracking.
Solar cells employed in space applications require protection from damaging effects of solar radiation. Radiation damage-induced degradation of the power output of silicon solar arrays is well-known. One approach to reduce this problem is through the use of gallium arsenide (or other III-V compound semiconductor) solar cells. Such III-V devices evidence a superior radiation hardness as compared to silicon solar cells. However, such III-V devices are also more expensive, as compared with silicon solar cells.
Cover glasses are commonly used to shield the solar cells from part of the damaging radiation. However, the effectiveness of such cover glass protection is limited, and leads either to solar cell life limitations because the cover glasses have to be thin or to weight penalties if the cover glasses are made too thick. Further, such cover glasses fail to provide any concentration of the solar radiation. The concentration ratio is thus 1.times. and no savings in cell cost is evidenced.
Concentrators have been developed for space applications. One such example is discussed in U.S. Pat. No. 4,494,302, which uses a Cassegrain mirror concentrator. A solar cell is exposed to concentrated sunlight in the Cassegrain mirror concentrator in which the light is focused by a secondary mirror on the solar cell attached to the front of a primary mirror. The back of the primary mirror is made black, and thus serves as a radiator, dissipating the heat from the solar cell. However, such concentrators suffer from a tracking problem, in that with concentrators of large ratio, the image easily moves out of the focal point as the sun's position moves away from the normal to the aperture plane of the module. For example, for a 2-D ideal concentrator trough providing a concentration ratio of 500.times., a shift in position of the sun by only 0.1.degree. from normal is all that is necessary for the image to be out of the focal point. Thus, acceptance angle of any concentrator must be optimized so as to minimize the output power losses caused by tracking errors.
Trough-shaped, non-imaging, ideal concentrator elements are known for concentrating solar radiation onto earth-based solar cells and for increasing acceptance angle; see, e.g., U.S. Pat. No. 4,964,713 to Goetzberger, and U.S. Pat. Nos. 3,923,381 and 4,114,592, both to Winston. Such concentrator elements may comprise two stages. The Winston patents disclose a first stage comprising a trough with parabolic sides which are mirrored and a second stage comprising a dielectric material using total internal reflectance (TIR). The Goetzberger patent discloses a first stage comprising a trough with parabolic sides in which the trough comprises glass or plastic and a second, smaller stage comprising either the same or a different material, also having parabolic sides.
Goetzberger's trough of glass or plastic adds undesirable weight for space applications, while for practical purposes, Winston's combination only achieves a concentration of about 5 times (5.times.) and requires an undesirably long distance from the entrance of the first stage to the surface of the solar cell.
Thus, a concentrator that provides excellent off-normal performance, reduced focal distance (hence, reduced mass), high concentration, and light weight is desired.